1. Field of the Invention
The present invention relates to a composite pressure-sensitive adhesive tape and to a solventless method for making these tapes.
2. Description of the Prior Art
Composite pressure-sensitive adhesive tapes are known. Typically they comprise at least one adhesive layer adhered to another layer. This other layer may comprise a second layer of adhesive or a backing layer which may itself comprise several layers of material.
Techniques for the manufacture of composite pressure-sensitive adhesive tapes are also known. One technique comprises a multistep solvent coating process. Another comprises a multistep hot melt coating process. In each of these techniques the individual layers that make up the composite tape are applied sequentially. Consequently, they are time consuming and labor intensive. Solvent coating processes further necessitate the use of special handling and equipment so as to minimize hazards to workers and the environment. Both processes subject the tape to severe operating conditions (for example elevated temperature, solvent atmosphere, etc.). This necessitates the use of non-extensible, tear-resistant, heat-resistant and/or water resistant backing materials so as to withstand the rigors of the manufacturing process. However, this also limits the use of polymeric materials as the backing material to those that are oriented and/or are sufficiently thick to withstand the rigors of the manufacturing process.
An attempt to reduce the number of steps in the process involves the use of coextrusion. Such techniques are described in a number of publications. For example, U.S. Pat. No. 3,415,920 discloses the manufacture of a polyolefin encapsulated adhesive fiber by the coextrusion of an adhesive and an outer skin of polyolefin.
Japanese patent publication JP-46-21120 discloses a process for the melt extrusion of a stream of thermoplastic synthetic resin and a stream of an adhesive, guiding the two streams of resin through a single die to form a single fluid flow having separate layers made up of each stream, and forming a tape having adhesive on one side.
U.S. Pat. No. 3,823,211 discloses a method of coextruding an adhesive/backing construction in the form of a tubular film and a method of forming a bubble of the backing material followed by coating an adhesive on the inside surface of the bubble of the backing material. Both techniques result in tape having adhesive on one side.
U.S. Pat. No. 4,214,261 discloses a multilayer polypropylene film which can be prepared by coextrusion. It further discloses that an adhesive can be subsequently coated onto the multilayer film.
U.S. Pat. No. 4,379,806 discloses producing a two-layer pressure-sensitive adhesive tape. In one process a tacky, thermoplastic rubber/resin pressure-sensitive adhesive and a backing of a normally non-tacky thermoplastic resin are coextruded to provide a tape having adhesive on only one side. In another method a blown layered film of the backing and adhesive is extruded. The film is then collapsed so that the backing film is joined to itself. The adhesive and the backing each have a torque value of between about 100 meter grams and 1000 meter grams when worked at 74 rpm at 420.degree. F. in a Brabender torque dynamometer. Additionally, the adhesive is joined to the backing "through an intermediate interlocking layer formed by controlled penetration of components during coextrusion of the film-forming and the adhesive compositions" (Col. 2, lines 3-7).
U.S. Pat. No. 4,888,075 discloses an all plastic multilayer liner and facestock for pressure-sensitive labels, tapes, etc. and a method of making such liners and facestocks. The liner/facestock is coextruded from any suitable coextrusion die such as, for example, a Cloeren "vane" die (Col. 2, lines 56-61). The pressure-sensitive adhesive is subsequently applied to the liner/facestock either by adhesive transfer from a release liner (Col. 6, lines 14-22), or in a subsequent coating step (Col. 6, lines 23-29 and FIG. 3A).
European Patent Application Publication No. 0 411 820 A1 discloses a reinforced pressure-sensitive adhesive tape and a method of making the same. The tape comprises a backing having a plurality of ribs at least partially embedded in one of its major surfaces. A pressure-sensitive adhesive is provided on at least one surface of the backing. The backing and reinforcing ribs can be coextruded together followed by application of the adhesive.
There have also been attempts to coextrude multilayer two-sided adhesive tapes directly into a nip formed between two solid surfaces. This technique has not been entirely satisfactory as the pressure on the extrudate while it is in the nip causes the adhesive to aggressively adhere to one of the surfaces. Once adhered, it is difficult to remove the adhesive.
While the tapes of these prior efforts are useful, their manufacture and conversion into an adhesive tape generally involves a series of sequential operations or extra handling which add to the cost and complexity of their preparation. The tapes disclosed in the above-mentioned publications have adhesive on only one side. To the extent that they disclose a tape having adhesive on both sides, they do so only through a multi-step process, rather than one in which the entire tape is produced via a single melt processing step.